Air conditioners suitable for vehicles and methods for operating such air conditioners

ABSTRACT

An air conditioner ( 1 ) includes an air conditioning circuit ( 2 ) in which a cooling medium circulates. An electrically powered compressor (C) is disposed within the air conditioning circuit ( 2 ) for compressing the cooling medium and discharging the cooling medium under high pressure. A refrigerant superheat feedback device ( 22 ) adjusts the superheat condition of the cooling medium that is returned to the compressor (C) for compression in order to ensure an adequate supply of lubrication oil to the compressor (C).

[0001] This application claims priority to Japanese application serial number 2001-43975, which application is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to air conditioners for vehicles and methods for operating such air conditioners. In particular, the present invention relates to air controlling techniques in such air conditioners that have an air control circuit and a compressor, preferably an electrically driven compressor, for circulating a cooling medium or refrigerant within the air control circuit.

[0004] 2. Description of the Related Art

[0005] Generally speaking, known air conditioning compressors are mechanically driven by means of a belt coupled to the engine, as in U.S. Pat. No. 5,813,249. However, electrically driven compressors have been proposed for use as air conditioner compressors, because the rotational speed of the electric motor can be controlled independently of the rotational speed of the engine.

[0006] In addition, lubrication oil is typically utilized in order to lubricate sliding parts within the compressor. As a result, lubrication oil recovering devices are usually disposed within the compressor in order to recover the lubrication oil and prevent the lubrication oil from flowing out of the compressor housing into the air conditioning circuit (e.g., into the condenser and evaporator). However, incorporation of such lubrication oil recovering devices increases the manufacturing costs and size of such compressors and thus, elimination of such lubrication oil recovering devices would be advantageous.

SUMMARY OF THE INVENTION

[0007] It is, accordingly, one object of the present invention to teach improved air conditioners that utilize compressors that do not require lubrication oil recovering devices. Therefore, manufacturing costs of such air conditioners can be reduced, and the size of the compressors can be minimized.

[0008] In one aspect of the present teachings, air conditioners are taught that include an air conditioning circuit in which a cooling medium circulates. A compressor may be disposed within the air conditioning circuit and preferably serves to compress the cooling medium and discharge the cooling medium under higher pressure. Thus, the compressed cooling medium can then be expanded, e.g., in an evaporator, in order to cool a flow of air that will be supplied to the vehicle interior. Preferably, the compressor may comprise an electrically driven motor that drives the compressor. A refrigerant superheat feedback device may preferably vary the degree of superheat or the superheat condition of the cooling medium that is returned to the compressor. For example, a superheat monitoring device may be disposed downstream of an evaporator in order to monitor the superheat condition of the cooling medium that is being returned to the compressor for compression. Based upon the detected superheat condition of the cooling medium downstream of the evaporator, the flow of cooling medium into the evaporator can be appropriately adjusted, as will be discussed further below.

[0009] According to the present specification, the term “superheat” or “degree of superheat” is intended to mean the difference (usually, measured in degrees of Celsius or Fahrenheit) between the actual temperature of the cooling medium (refrigerant), which actual temperature is measured at a certain pressure, and the saturation temperature of the cooling medium (refrigerant) at that same pressure. In other words, the degree of superheat of the cooling medium (refrigerant) may be expressed as the difference between the vapor point of the cooling medium at a certain pressure (i.e., the temperature at which the cooling medium evaporates at a given pressure) and the actual temperature of the cooling medium exiting the evaporator. Thus, if the cooling medium (refrigerant) is at a higher temperature when exiting the condenser than the vapor saturation temperature for the pressure at which the cooling medium is exiting the condenser, the difference is called the degree of superheat or the superheat condition of the cooling medium (refrigerant).

[0010] Therefore, the refrigerant superheat feedback device may effectively control the superheat condition of the cooling medium that is being supplied to the compressor. For example, at relatively low pressures and high temperatures, the cooling medium is substantially in a gaseous state. On the other hand, at relatively high pressures and low temperatures, the cooling medium is substantially in a liquid state. Naturally, at intermediate pressures and temperatures, the cooling medium may be in a substantially dual-phase gas-liquid state.

[0011] In one representative embodiment, the refrigerant superheat feedback device can adjust the superheat condition of the cooling medium, so that the cooling medium exiting the evaporator is in a dual-phase state. In that case, the liquid phase of the cooling medium can effectively convey lubrication oil into the compressor and reliably lubricate sliding parts within the compressor. Thus, if the cooling medium can be effectively maintained in a state that will ensure effective lubrication of the compressor regardless of the workload on the compressor, the air conditioner will not require a costly lubrication oil recovering device. In addition, the air conditioner may have a relatively simple construction as compared to known air conditioning systems.

[0012] Thus, the inventors have found that the lubrication oil that flows out from the compressor can still be used to lubricating part within the compressor without incorporating lubrication oil recovering devices, if the lubrication oil adequately circulates within the air conditioning circuit and returns to the compressor. In order to effectively circulate the lubrication oil, the cooling medium may serve as a carrier for the lubrication oil. The ability of the cooling medium to serve as a carrier for the lubrication oil may be improved by controlling the degree of superheat of the cooling medium. For example, saturated cooling medium having a liquid phase of the cooling medium may effectively convey the lubrication oil even if the flow rate of the cooling medium is relatively small, which may occur in a low load operation for the air conditioning system. On the other hand, if the compressor is operating under a relatively high load, the flow rate of the lubrication oil through the air conditioning system will be relatively high. In that case, an adequate amount of lubricating oil will be returned to the compressor, even if the cooling medium is substantially in a gaseous state (i.e., the cooling medium returning to the compressor contains little or no liquid cooling medium).

[0013] In another aspect of the present teachings, methods for operating air conditioners are taught that include adjusting the superheat condition of the cooling medium that is supplied to the compressor in response to the load that is applied to the air conditioner. Therefore, if the superheat condition of the cooling medium, which is supplied to the compressor, is adjusted in response to the load that is applied to the air conditioner, the cooling medium may be brought into a dual-phase state, which includes a liquid phase of the cooling medium, in order to more effectively convey the lubrication oil within the air conditioning system.

[0014] Other objects, features and advantages of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic diagram of a representative air conditioner;

[0016]FIG. 2 shows a representative cross-charge expansion valve and associated parts;

[0017]FIG. 3 shows the relationship between valve lift and the flow rate of cooling medium (refrigerant) compared to the enthalpy of the cooling medium (refrigerant);

[0018]FIG. 4 is a graph showing the relationship between temperature T(12) and pressure P(12) at an outlet of an evaporator of the air conditioner when a representative expansion valve is incorporated; and

[0019]FIG. 5 is a Mollier chart for a cooling medium circulation process of the air controlling circuit.

DETAILED DESCRIPTION OF THE INVENTION

[0020] In one embodiment of the present teachings, the refrigerant superheat feedback device may vary the state (e.g., dual-phase state or substantially gaseous state) of the cooling medium that returns to the compressor during the circulating process in response to the load applied to the air conditioner (e.g., the compressor) during the air conditioning operation. The refrigerant superheat feedback device preferably performs two functions: (1) monitoring the superheat condition (e.g., the enthalpy) of the cooling medium that is exhausted from an evaporator and which cooling medium will be supplied to the compressor and (2) adjusting the flow of cooling medium into the evaporator in order to maintain an appropriate state of the cooling medium that is exhausted from the evaporator.

[0021] For example, if a high load is being applied to the air conditioning system, the cooling medium that is being exhausted from the evaporator may be maintained in a substantially gaseous state, thereby transferring the maximum amount of cooling energy to a flow of air that will be supplied to the vehicle interior. In this case, because the flow rate of the cooling medium within the air conditioning system is relatively high, sufficient lubricating oil will be circulated to the compressor in order to reliably lubricate the compressor parts, even though the cooling medium is substantially in a gaseous state.

[0022] On the other hand, if the load on the air conditioning system is relatively low, the flow rate of the cooling medium within the air conditioning systems also may be relatively low. Because gaseous cooling medium is less effective for conveying lubricating oil than liquid cooling medium, the compressor may not be adequately lubricated if only gaseous cooling medium is being supplied to the compressor in a low load operation. Therefore, the superheat state of the cooling medium exiting the evaporator can be adjusted by changing the flow of cooling medium into the evaporator in order to ensure that dual-phase cooling medium is exhausted from the evaporator and is conveyed to the compressor during a low load operation.

[0023] Because the dual-phase cooling medium includes a liquid phase that can effectively convey the lubricating oil, adequate lubrication of the compressor can be ensured, even in low load operations. Consequently, it is not necessary to utilize a lubricating oil recovery device within the air conditioning system, because an adequate supply of lubricating oil to the compressor is ensured during all types of workload on the air conditioning system (i.e., the compressor).

[0024] In one representative embodiment, the refrigerant superheat feedback device may include a control valve disposed within the air control circuit, or any other type of controller, that is coupled to the air conditioning circuit but is physically isolated from the cooling medium within the air conditioning circuit. For example, an expansion valve may be utilized to control the flow rate of the cooling medium in response to changes in the load applied to the air conditioning system. Alternatively, the refrigerant superheat feedback device may be a separate control valve. Also, the combination of these valves may be used.

[0025] In addition, the refrigerant superheat feedback device may include a device that monitors the superheat condition of the cooling medium exiting the evaporator and adjusts the flow rate of cooling medium into the evaporator. For example, in one representative embodiment, a cross-charge type expansion valve may be utilized for this purpose. Generally speaking, such a device includes two features.

[0026] First, a means for monitoring the temperature of the cooling medium is provided. For example, a substantially sealed volume of gas, which gas preferably has a composition that differs from the cooling medium, may be disposed substantially adjacent to the portion of the air conditioning circuit containing the cooling medium that has been exhausted from the evaporator. Thus, because this gas is physically isolated from the cooling medium, but is disposed in a manner so as to have substantially the same temperature as the cooling medium, the gas will expand and contract as the temperature of the cooling medium respectively increases and decreases.

[0027] Second, a means for monitoring the pressure of the cooling medium also is preferably provided. For example, a movable diaphragm may separate the cooling medium within the air conditioning system and the gas within means for monitoring the temperature of the cooling medium. Thus, as the relatively pressures of the gas and the cooling medium change, the diaphragm will change position. If the diaphragm is coupled to the expansion valve, the change in position of the diaphragm will change the opening degree of the expansion valve. Therefore, the superheat condition of the cooling medium that is being exhausted by the evaporator is reflected by the position of the diaphragm. Further, the position of the diaphragm determines the flow rate of the cooling medium into evaporator.

[0028] Consequently, the superheat condition of the cooling medium that is exiting the evaporator can be effectively “fed back” to the expansion valve in order to control the opening degree of the expansion valve. Thus, the flow rate of the cooling medium into the evaporator also can be effectively controlled in order to maintain the state of the cooling medium that is exiting the evaporator in a condition that will effectively convey sufficient lubricating oil to the compressor and ensure adequate lubrication of the compressor.

[0029] In another representative embodiment, when the load on the air conditioning system is relatively low and the flow rate of circulating cooling medium is relatively small, the refrigerant superheat feedback device may decrease the superheat condition (e.g., enthalpy) of the cooling medium that is supplied to the compressor. In this case, the gaseous cooling medium may be brought into a dual-phase state that includes a liquid phase. Therefore, the lubrication oil may be conveyed by the liquid phase of the cooling medium so as to reliably circulate and return to the compressor. As a result, the lubrication of parts within the compressor may be reliably maintained, and the durability of the compressor may be improved.

[0030] Thus, the lubrication oil can be effectively circulated in a cost-effective and simple manner by incorporating such a refrigerant superheat feedback device and the air conditioner will not require a costly lubrication oil recovering device.

[0031] In another representative embodiment, the refrigerant superheat feedback device, which may include an expansion valve, may serve to cause the cooling medium that is being returned to the compressor to be substantially a vapor (i.e., substantially gaseous state) when the load applied to the air conditioner is high and the flow rate of circulating cooling medium is relatively large. On the other hand, when the load applied to the air conditioner is low and the flow rate of circulating cooling medium is relatively small, the expansion valve may serve to cause the cooling medium that is being returned to the compressor to be dual-phase (i.e., gas-liquid).

[0032] Therefore, when the load applied to the air conditioner is high and the flow rate of circulating cooling medium is relatively large, the superheat condition of the cooling medium may be increased, which will cause the cooling medium to be in a substantially gaseous state. However, because the flow rate of the gaseous cooling medium is relatively large, the lubrication oil still may reliably flow together with the cooling medium. Therefore, the lubrication oil will smoothly and adequately circulate within the air control circuit.

[0033] As a result, the lubrication oil can be effectively circulated in a cost-effective and simple manner by incorporating a refrigerant superheat feedback device having the above features and the air conditioner does not require a costly lubrication oil recovering device. Furthermore, the compressor can be effectively operated regardless of the workload on the air conditioning system.

[0034] In another representative embodiment, methods for operating an air conditioner are taught that may include adjusting the superheat condition of the cooling medium in response to a load that is applied to the air conditioner. For example, if the load applied to the air conditioner is low and the flow rate of circulating cooling medium is relatively small, the superheat condition of the cooling medium, which is being returned to the compressor, may be controlled such that the cooling medium is brought to a dual-phase state.

[0035] In that case, the vapor of the cooling medium will partially liquefied. As a result, even if the lubrication oil within the compressor flows into the air control circuit when the load applied to the air conditioner is low and the flow rate of circulating cooling medium is small, the lubrication oil may flow together with the liquefied phase of the vapor and then may return to the compressor. Consequently, the lubrication oil can be effective circulated in a cost-effective and simple manner.

[0036] Each of the additional features and method steps disclosed above and below may be utilized separately or in conjunction with other features and method steps to provide improved air conditioners and methods for designing and using such air conditioners. Representative examples of the present invention, which examples utilize many of these additional features and method steps in conjunction, will now be described in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed in the following detail description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention.

[0037] A representative embodiment of an air conditioner will now be described with reference to the drawings. A schematic diagram of the general configuration of a representative air conditioner 1 is shown in FIG. 1. The air conditioner 1 may include an air controlling circuit 2 that serves to circulate cooling medium or refrigerant. An electrically driven compressor C, a condenser 10, an evaporator 12, a receiver 14 and an expansion valve 20 may be disposed within the air controlling circuit 2. The compressor C preferably serves to compress a gaseous, or substantially gaseous, cooling medium and discharge pressurized cooling medium. An inverter I may be included to selectively power an electric motor M that drives the compressor C. In one preferred embodiment, the compressor C may be a scroll-type compressor.

[0038] A vehicle engine E may serve as the drive source of a vehicle and may be mechanically connected to an alternator O, e.g., by a belt or another transmission means. The alternator O may be electrically connected to a battery B and also to the inverter I. Therefore, electric current generated by the alternator O may be utilized to drive the motor M or may charge the battery B.

[0039] The expansion valve 20 preferably serves as a pressure reducer or regulator by rapidly expanding the relatively high temperature, high-pressure liquid refrigerant supplied by the condenser 10. By passing the liquid refrigerant, e.g., through a small opening (not shown) in the expansion valve 20, a relatively low temperature, low-pressure gas-liquid two-phased atomized refrigerant may be generated.

[0040] A thermosensitive cylinder or element 22 may be utilized to essentially “feedback” the superheat condition of the cooling medium at the exhaust port of the evaporator 12 to the evaporation valve 20 in order to control the supply of refrigerant to evaporator 12. The thermosensitive cylinder 22 preferably contains a gaseous composition that is different from the cooling medium or refrigerant that is disposed within the air conditioning circuit 2. Moreover, the gas within the thermosensitive cylinder 22 is preferably isolated from the refrigerant within the air conditioning circuit 2. In other words, the thermosensitive cylinder 22 is disposed so as to adjoin or substantially contact the portion of the air conditioning circuit 2 containing the refrigerant that has been exhausted from the evaporator 12. However, the gas within the thermosensitive cylinder 22 and the refrigerant within the air conditioning circuit 2 remain separated. Therefore, the thermosensitive cylinder 22 may serve as a refrigerant temperature detector that detects the temperature of the gaseous refrigerant that is being fed into the compressor C after having been exhausted from the evaporator 12. In order words, the gas within the thermosensitive cylinder 22 preferably assumes the same temperature as the cooling medium exiting the evaporator 12, due to the proximal relationship of the thermosensitive cylinder 22 and the air conditioning circuit 2.

[0041] Referring to FIG. 2, the expansion valve 20 may be a cross-charge expansion valve and may include a throttle valve 21 that is disposed at the inlet of the expansion valve 20. A spring 23 may bias the throttle valve 21. Further, the throttle valve 21 may connected to a diaphragm 25 that is disposed within a diaphragm chamber 27. A first side of the diaphragm chamber 27 may communicate with the thermosensitive cylinder 22 via a first tube 29 (i.e., the gas within the thermosensitive cylinder 22 applies pressure to the first side of the diaphragm 25). A second side of the diaphragm chamber 27 may communicate with the outlet side of the evaporator 12 via a second tube 31 (i.e., the cooling medium within the air conditioning circuit 2 applies pressure to the second side of the diaphragm 25). As a result, the position of the throttle valve 21 (i.e., the degree of opening) will vary in response to differences between the pressure within the first tube 29 and the pressure within the second tube 31, which differences will affect the relative position of the diaphragm 25.

[0042] In other words, the second tube 31 is disposed in a way that it circumvents the evaporator 12 and forms a pressure guiding passage which connects the interior of the thermosensitive cylinder 22 with the interior of a pressure chamber provided at one side of the diaphragm 25. The first tube 29 serves as pressure communication unit for communicating pressure changes within the thermosensitive cylinder 22 to the pressure chamber provided at the other side of the diaphragm 25.

[0043] Preferably, activated carbon CA may be contained within the thermosensitive cylinder 22. As noted above, in a cross-charge expansion valve, the gas disposed within the thermosensitive cylinder 22 is different in kind or composition from the refrigerant flowing through the air controlling circuit 2. However, this different gas is sealed within a channel that connects the diaphragm chamber 27 and the thermosensitive cylinder 22, as discussed above. The gas within this channel is chosen such that at least some of the gas is absorbed by the activated carbon CA in order to provide a reservoir of gas for expansion, when the temperature of the cooling medium that is being discharged from the evaporator 12 increases.

[0044] The thermosensitive cylinder 22 may be attached to the outlet of the evaporator 12, so that the pressure within the thermosensitive cylinder 22, as well as the pressure within the first tube 29, varies in response to the superheat condition of the refrigerant at the outlet of the evaporator 12. For example, the amount of absorption of the gas by the activated carbon may increase as the temperature at the outlet of the evaporator 12 decreases. Therefore, the pressure within the thermosensitive cylinder 22 may vary with changes in the temperature at the outlet of the evaporator 12. As a result, the opening degree of the throttle valve 21 of the expansion valve 20 may be controlled in response to the difference between the pressure of the gas within the thermosensitive cylinder 22 and the pressure of the refrigerant at the outlet of the evaporator 12.

[0045] Preferably, if the superheat condition of the refrigerant at the outlet of the evaporator is too high during a low load condition (i.e., the refrigerant is in a substantially gaseous state during a low load operation), the degree of opening of the throttle valve 21 may be increased in order to increase the flow rate of the refrigerant into the evaporator. As a result, the refrigerant is prevented from reaching an excessive superheat condition. FIG. 3 shows a schematic graph showing the relationship between the lift of the throttle valve 21 and the pressure difference (ΔP) between the pressure within the first tube 29 (or the pressure within the upper side of the diaphragm chamber 22) and the pressure at the outlet of the evaporator 12 (or the pressure within the lower side of the diaphragm chamber 22). In FIG. 3, P0 is a predetermined value at which the valve 21 starts to open. Because cross-charge type expansion valves are well known in the art, further details concerning the construction of the expansion valve 20 are not necessary.

[0046] A representative method for operating the air conditioner 1 will now be described with reference to FIGS. 1 to 5. An explanatory graph showing the characteristics of a cross charge type expansion valve is shown in FIG. 4. A Mollier chart for a cooling medium circulation process of the air controlling circuit is shown in FIG. 5.

[0047] Referring to FIG. 5, the relationship between pressure P and enthalpy h for a phase change during the cooling medium circulation process generally may be represented by the Mollier chart shown in FIG. 5. In a phase change, as from a liquid to a gas, the change in enthalpy of the system is the latent heat of vaporization. As the air conditioner 1 is operated to drive the compressor C, saturated refrigerant vapor A1′ within the air conditioning circuit may be drawn into and adiabatically compressed by the compressor C and then may be discharged into the air conditioning circuit 2 as a superheated refrigerant vapor A2′ that has a relatively high temperature and high pressure.

[0048] The superheated refrigerant vapor A2′ discharged from the compressor C may be isobarically cooled (Q (10)) and be liquefied within the condenser 10 and the receiver 14 so as to become a liquid cooling medium A3. That is, the vapor A2′ is cooled without changing pressure. The liquid cooling medium A3 may then be expanded by the expansion valve 20 and the evaporator 12 in order to generate a gas-liquid refrigerant vapor A4. The gas-liquid vapor A4 may subsequently flow into the evaporator 12 and cool the air passing across the evaporator 12 (which cooled air will be supplied to the vehicle interior) through heat exchange between the gas-liquid vapor A4 and the air. Hence, the gas-liquid vapor A4 absorbs energy from the air (Q (12)), which causes the liquid content of the gas-liquid vapor A4 to isobarically vaporize. As a result, the gas-liquid vapor A4 may become a substantially saturated vapor A1′, which is again suctioned into and pressurized by the compressor C.

[0049] In this embodiment, the gas/liquid state of the cooling medium at the outlet of the evaporator 12 may vary in response to the degree of valve opening of the expansion valve 20. For example, although the compressor C may compress the saturated refrigerant vapor A1′ into the superheated refrigerant vapor A2′, the compressor also may compress superheated refrigerant vapor AI into superheated refrigerant vapor A2. That is, the degree of valve opening of expansion valve 20 will determine the enthalpy h of the refrigerant exhausted from the evaporator 12, and thus the state of the refrigerant that is supplied to the compressor C. Because a sufficient amount of lubrication oil must always be supplied to the operating compressor C and liquid refrigerant conveys lubrication oil more readily than gaseous lubrication oil, it is advantageous to control the state of the refrigerant exhausted from evaporator 12 and drawn into compressor C in order to ensure that compressor C is adequately lubricated.

[0050] By incorporating the cross-charge expansion valve 20 in the representative embodiment, the superheat condition SH1 of the cooling medium may vary in response to the temperature T(12) at the outlet of the evaporator 12. In contrast, when a superheated (SH) expansion valve is used instead of the expansion valve 20, the superheat condition SH2 of the cooling medium may be a fixed value irrespective of changes in the temperature T(12).

[0051] When the amount of energy Q(12) to be exchanged between the cooling medium and the conditioning air becomes greater (i.e., the air controlling load is increased) during the circulation of cooling medium in the air controlling circuit 2, the relative amount of vaporized cooling medium within the evaporator 12 may increase. Therefore, the temperature T(12) at the outlet of the evaporator 12 may increase. According to the representative embodiment, the superheat condition SH1 of the cooling medium may increase as the temperature T(12) increases. As a result, the superheated cooling medium may return to the compressor C.

[0052] On the other hand, when the amount of energy Q(12) to be exchanged between the cooling medium and the conditioning air becomes less (i.e., the air controlling load is decreased) during the circulation of cooling medium, the amount of heat that may be absorbed by the conditioning air from the cooling medium flowing through the evaporator 12 may decrease. Therefore, the temperature T(12) at the outlet of the evaporator 12 may decrease. According to the representative embodiment, the superheat condition SH1 of the cooling medium may decrease as the temperature T(12) decreases. As a result, the cooling medium at the outlet of the evaporator 12 may not be completely vaporized (i.e., the cooling medium will be in a dual gas-liquid state).

[0053] As discussed above, lubrication oil is disposed within the cooling medium in order to reliably lubricate sliding parts within the housing of the compressor C. Known compressors generally utilize a lubrication oil recovering device for preventing the lubrication oil from leaking out into the air conditioning circuit along with the cooling medium that is discharged from the compressor housing. For example, in an air conditioning circuit incorporating a SH-type expansion valve, the cooling medium that returns to the compressor is always in the state of a heated vapor. However, when the flow rate of the cooling medium decreases during a low load operation of the air conditioner, the lubrication oil may not properly circulate with the relatively low flow of the gaseous cooling medium. Thus, the compressor C may not be properly lubricated.

[0054] According to the representative air conditioner 1, the electrically powered compressor C does not require a lubrication oil recovery device. Instead, the cross-type expansion valve 20 may be incorporated to reliably circulate the lubrication oil during all workloads on the compressor C. In other words, the cross-type expansion valve 20 ensures that the cooling medium is always at a proper condition (i.e., dual-phase or substantially gaseous) in order to reliably supply lubrication oil to the moving parts within the compressor C while also sufficiently cooling the air that will be supplied to the vehicle interior. Thus, the expansion valve 20 may serve to vary the superheat condition of the cooling medium in response to the operation load applied to the air conditioner 1. In particular, when the air conditioner 1 operates under a relatively low load, in which the lubrication oil may not smoothly circulate, the expansion valve 20 may serve to provide a partially liquefied refrigerant vapor (i.e., a dual phase gas-liquid) at the outlet of the expansion valve. Thus, a dual phase refrigerant may be returned to the compressor C.

[0055] Therefore, the lubrication oil may reliably return to the compressor C along with the flow of the liquefied cooling medium. As a result, the circulation properties of the lubrication oil may be improved with respect to known air conditioners incorporating SH-type expansion valves, in particular during a low load operation of the air conditioner 1. Although the temperature of the cooling medium at the outlet of the expansion valve 20 may increase during the high load operation, this may not cause any problem, because the lubrication oil may flow along with the gaseous cooling medium that flows at a higher rate. Consequently, the circulation properties of the lubrication oil in the air conditioning circuit 2 may be improved.

[0056] According to the representative embodiment, when the air conditioner is operated under low load and the flow rate of circulation of the cooling medium is relatively small, the cross-charge expansion valve 20 may decrease the degree of superheat of the cooling medium that returns to the compressor C. In that case, the cooling medium may be brought into a saturated state or a partly liquefied state, which state may improve the circulating properties of the cooling medium and the lubrication oil. Therefore, air conditioners having improved circulating properties can be easily attained at a lower cost by incorporating the representative expansion valve 20 in place of a known lubrication oil recovery device.

[0057] The present teachings should not be limited to the representative embodiment, but instead, may be used for different applications and may be modified in various ways. For example, the cross-charge type expansion valve 20 may be replaced with another device or devices that are capable of causing the superheat condition of the cooling medium that returns to the compressor C to appropriately change in order to supply adequate lubrication oil to the compressor. For example, expansion valves of different types or control valves can be advantageously utilized to vary the cross-sectional area of the flow line in the air conditioning circuit to control the refrigerant temperature. 

1. An air conditioner comprising: an air conditioning circuit comprising a circulating cooling medium and lubricating oil, an electrically powered compressor disposed within the air conditioning circuit, the compressor being arranged and constructed to compress the cooling medium and discharge the cooling medium under high pressure, and a refrigerant superheat feedback device arranged and constructed to adjust the superheat condition of the cooling medium that is returned to the compressor in order to ensure an adequate supply of lubrication oil to the compressor during a low load operation.
 2. An air conditioner as in claim 1, wherein the refrigerant superheat feedback device comprises an expansion valve.
 3. A method for compressing a cooling medium using an air conditioner that includes an electrically-driven compressor, which is arranged and constructed to compress the cooling medium and discharge the cooling medium under high pressure, comprising: adjusting the superheat condition of the cooling medium that is returned to the compressor in response to a load that is applied to the air conditioner.
 4. A method as in claim 3, wherein the change of the cooling medium temperature is performed by an expansion valve.
 5. An air conditioner comprising: an air conditioning circuit comprising a circulating cooling medium and lubricating oil, an electrically driven compressor disposed within the air conditioning circuit, the compressor being arranged and constructed to compress the cooling medium and discharge the cooling medium under high pressure, and a controller arranged and constructed to control the phase state of the cooling medium within the air conditioning circuit, wherein the controller causes the cooling medium supplied to an inlet of the compressor to be in dual-phase, gas-liquid state when an operating load on the air conditioner is relatively low.
 6. An air conditioner as in claim 5, further including an evaporator disposed on an upstream side of the compressor and within the air conditioning circuit, wherein the controller is further arranged and constructed to control the flow of the cooling medium at an inlet of the evaporator.
 7. An air conditioner as in claim 6, wherein the controller controls the flow of the cooling medium at the inlet of the evaporator based upon the superheat condition of the cooling medium at the outlet of the evaporator.
 8. An air conditioner as in claim 6, wherein the controller comprises a thermosensitive cylinder disposed proximal to the air conditioning circuit downstream of the evaporator and communicating with a first side of a movable diaphragm, and the air conditioning circuit communication with a second side of the movable diaphragm, the movable diaphragm being coupled an expansion valve disposed upstream of the evaporator, the thermosensitive cylinder containing a gaseous composition that is different from the cooling medium, wherein the expansion valve is arranged and constructed to control the flow of the cooling medium into the inlet of the evaporator based upon differences between the pressure of the cooling medium at the outlet of the evaporator and the pressure of the gaseous composition within the thermosensitive cylinder.
 9. An air conditioner as in claim 6, wherein the controller comprises a cross-charge expansion valve.
 10. An apparatus suitable for circulating cooling medium and lubrication oil to an electrically powered compressor within an air control circuit of an air conditioner, comprising: a controller arranged and constructed to control the phase state of the cooling medium in the air conditioning circuit to as to supply a dual-state gas liquid cooling medium to an inlet of the compressor during operation of the air conditioner under a low load.
 11. An apparatus as in claim 10, further comprising an evaporator disposed on an upstream side of the compressor and within the air conditioning circuit, wherein the controller is arranged and constructed to control the flow of the cooling medium into an inlet of the evaporator.
 12. An apparatus as in claim 11, wherein the controller is further arranged and constructed to control the flow of the cooling medium into the inlet of the evaporator based upon the superheat condition of the cooling medium exiting an outlet of the evaporator.
 13. An apparatus as in claim 12, wherein the controller comprises a cross-charge expansion valve for controlling the flow of cooling medium into the inlet of the evaporator.
 14. A method for circulating a refrigerant and lubrication oil within an air conditioning circuit of an air conditioner including an electrically powered compressor, comprising: supplying the lubrication oil to the compressor within a dual-state gas-liquid cooling medium when the load on the air conditioner is relatively low.
 15. A method as in claim 14, further comprising controlling the phase state of the cooling medium at an inlet of the compressor based upon the pressure at an inlet of an evaporator that is disposed on an upstream side of the compressor within the air conditioning circuit.
 16. An air conditioner comprising: an electrically powered compressor arranged and constructed to compress a refrigerant and discharge the refrigerant under higher pressure, the refrigerant comprising lubricating oil for lubricating parts within the compressor, a condenser receiving the refrigerant from the compressor, an expansion valve receiving refrigerant from the condenser, an evaporator receiving refrigerant from the expansion valve, and means for adjusting the amount of refrigerant supplied to evaporator in order to adjust the phase state of the refrigerant that is returned to the compressor, thereby ensuring an adequate supply of lubrication oil to the compressor during a low load operation.
 17. An air conditioner as in claim 16, wherein the adjusting means contains a gas having a different composition from the refrigerant and the adjusting means is disposed proximal to an outlet port of the evaporator, wherein the gas within the adjusting means is physically isolated from the refrigerant, but assumes the same temperature as the refrigerant exiting the outlet port of the evaporator.
 18. An air conditioner as in claim 17, wherein the adjusting means further comprises a movable diaphragm coupled to the expansion valve, wherein the gas within the adjusting means communicates with a first side of the movable diaphragm and the refrigerant exiting from the outlet port of the evaporator communicates with a second side of the movable diaphragm, wherein changes in the relative pressures of the gas and refrigerant cause the movable diaphragm to move and adjust the amount of refrigerant supplied to the evaporator by the expansion valve.
 19. An air conditioner as in claim 18, wherein the adjusting means adjusts the phase state of the refrigerant returning to the compressor to be a substantially gas-liquid phase state when the compressor is operating under a relatively low load.
 20. A apparatus for circulating a refrigerant and lubrication oil within an air conditioning circuit of an air conditioner including an electrically powered compressor, comprising: means for supplying the lubrication oil to the compressor within a dual-state gas-liquid cooling medium when the load on the air conditioner is relatively low in order to ensure reliable lubrication of the compressor. 